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Fig. 1

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Chosen projection plane: the xy frame is centred on the X-ray source position X; the x-axis is the direction towards the optical candidate, located at point O. d is the angular distance between the two sources. This frame is useful at high declinations when we cannot consider the meridians any longer – the directions of the north pole in X and in O – to be parallel. It allows us to deal naturally with the poles.

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